1. Field of the Invention
The present invention relates to well sealing compositions and methods, and more particularly, but not by way of limitation, to such compositions and methods for performing sealing operations in wells.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in oil, gas and water well sealing applications during completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing procedures whereby pipes such as casing are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior of a pipe disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantial impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and seals the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
Hydraulic sealing compositions used for sealing subterranean formations or zones in wells must have particular properties. For example, the sealing compositions must be capable of being mixed and pumped without prematurely gelling, have sufficiently long pumping times to be placed in subterranean formations or zones, have high compressive and tensile strengths and have a high degree of resiliency after setting or hardening.
The American Petroleum Institute (API) has set standards for different classes for oil well cements to insure that the cement slurries formed with them have required properties. The API cements are Portland cements and because of the strict requirements placed on them, they are generally more difficult to produce and more expensive than other sealing compositions such as slag cement compositions.
Slag cement has heretofore been utilized to form hydraulic cement slurries used for cementing subterranean formations or zones in wells. Slag cement is produced from slag which is formed in the manufacture of iron. The iron is produced from iron ore and limestone flux in a blast furnace and slag is a by-product therefrom. The essential components of slag are the same oxides that are present in Portland cement, e.g., lime, silica and alumina, but their proportions are different. The slag comes from the blast furnace as a molten stream at a temperature of about 1400.degree. F. to 1500.degree. F. The slag is chilled very rapidly either by pouring it into a large excess of water or by contacting it with jets of water or a mixture of air and water. The quenching prevents the slag from rapidly crystallizing and causes it to solidify as a glassy material. Simultaneously, the quenching breaks up the material into small particles or grains. Granulated slag cement alone has a negligible cementing action until an alkaline activator is present, e.g. hydrated lime.
While a slag cement composition has substantial compressive strength after it sets, it generally includes extensive microcracks which decrease its flexural strength and causes it to be permeable to gasses and water. It has been shown that the hydration products of alkali activated slag contain an unstable gel phase which causes shrinking during setting and the formation of microcracks. That is, the alkali activated slag consists of two principal phases during hydration, one of which is calcium silicate hydrate. The other phase is rich in silica and has properties similar to silica gel. The silica phase is unstable and expels water resulting in irreversible shrinkage during setting. This shrinking produces the microcracks which in turn cause increased permeability and decreased tensile strength in the set slag cement.
Thus, there is a need for improved well sealing compositions and methods of using the compositions which are formed of relatively inexpensive slag cement, but which do not shrink and form micro cracks upon setting.